Switching system

ABSTRACT

An ATM switching system  1  is provided with an ATM switch  11,  a reserved connection memory  12  for storing reserved connection information, a call history memory  13  for maintaining call histories of requests for connection from subscriber&#39;s terminal units  2   1  to  2   n , and a call-signal processing section  15.  The call-signal processing section  15  generates a request for connection with respect to a trunk ATM switching network  3  by the use of the call histories in the call history memory  13  in the case where no call was issued from the subscriber&#39;s terminal units, and stores response results thereof in the reserved connection memory  16.  Thereafter, when there was a call from the subscriber&#39;s terminal units  2   −1  to  2   −n , and contents of the request for connection thereof are the same as the reserved connection information, which has been stored in the reserved connection memory  16,  processing for connection is executed by the use of the reserved connection information. As a result, an ATM switching system by which response becomes possible in even the case where a large amount of calls are issued at the same time, besides reduction in cost can also be attained is provided.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/674,872 filed Feb. 14, 2007, which is a continuation of U.S. patentapplication Ser. No. 11/223,143 filed Sep. 12, 2005, which is acontinuation of U.S. patent application Ser. No. 09/939,706 filed Aug.28, 2001, which issued as U.S. Pat. No. 6,961,308 on Nov. 1, 2005, thedisclosures of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to an ATM (Asynchronous Transfer Mode)switching system, and particularly to an ATM switching system that canperform ATM processing without delay in even a case where there are anumber of calls at the same time.

BACKGROUND OF THE INVENTION

ATM has such a characteristic that switching processing wherein all ofinformation are divided into a certain unit called by the name of “cell”a packet size of which is a fixed length (consisting of five byte headerand forty-eight byte data), and so divided information is transferredasynchronously can be carried out by only a hardware. Accordingly, suchswitching processing is suitable for treating unitarily multimediainformation. For instance, a frequency can be changed in response tocontents of data in accordance with such a manner wherein cells aretransferred thinly in case of E-mail, while cells are transferreddensely in case of voice or moving image. Thus, there is such acharacteristic that a large amount of data can be transmitted at a highspeed.

A procedure for communication in ATM switching system is specified indetail by ITU-TS (International TelecommunicationUnion-Telecommunication Standardization Section) being an internationalstandardization institution, ATM forum or the like. Such type of ATMswitching system is disclosed in, for example, Japanese Patent Kokai No.2000-49799. The constitution of such ATM switching system will bedescribed hereinafter by referring to the accompanying drawings whereinFIG. 1 is a block diagram showing a conventional ATM switching system.

In FIG. 1, an ATM switching system 100 is connected with subscriber'sterminal units 151 and 152. The ATM switching system 100 comprises anATM switch 101 for implementing switching processing, a multiplexer 102connected to an input stage of the ATM switch 101, a separator 103connected to an output stage of the ATM switch 101, an output line unitconnected to an output stage of the separator 103, a signal processor106 for inputting signals from the subscriber's terminal unit 151 to themultiplexer 102, a speech channel controller 107 connected to the ATMswitch 101, a call processor 108 connected to the speech channelcontroller 107, and a memory 109 connected to the call processor 108.

The signal processor 106 executes analytical processing for controlsignals such as call signals. The speech channel controller 107 controlsestablishment and release of an ATM connection based on call admissioncontrol (CAC) and call release control by means of the call processor108. The call processor 108 implements call admission control on thebasis of call signal and call release control on the basis of callreleasing signal. Furthermore, when congestion appears on a certainline, the call processor 108 interrupts call admission with respect tothe line. Namely, when a line becomes an over traffic state with respectto a transmission speed requested by return information (trafficinformation, bearer information and the like) relating to quality in acall signal (setup message), an establishment for ATM connection isrejected with respect to a call in question. Moreover, the memory 109retains a variety of data such as call processing data, and subscribers'data.

Then, a case where data is transmitted from the subscriber's terminalunit 151 to the subscriber's terminal unit 152 will be described herein.In this case, the ATM switch 101 routes cells delivered from thesubscriber's terminal unit 151 to the subscriber's terminal unit 152 bymeans of hardware switching to output the above-described cells to atransmission path connected to the subscriber's terminal unit 152. Inorder to realize such operation of the ATM switch 101 as describedabove, it is required to report information as to a counterpart to becommunicated, quality in communication, a communicating zone and thelike with respect to the ATM switching system and an ATM switchingnetwork. In this case, transmission for the report by an operator is acall.

Call processing in the ATM switching system 100 will be described. Whena call signal is delivered from the subscriber's terminal unit 151, thecall signal is input to the signal processor 106 through the input lineunit 104. Then, such analytical processing whether or not the callsignal has been composed in accordance with correct procedure andcontents is made by the signal processor 106. Thereafter, the callprocessor 108 processes adequancies of a variety of reported informationcontained in the call signal, and if a communication service based onthe reported information is permissible, the call processor makesrequired setting, so that a communication path is established via aroute of the input line unit 104→the multiplexer 102→the ATM switch101→the separator 103→the output line unit 105.

According to the conventional ATM switching system, however, the signalprocessor 106 and the call processor 108 are treated as a kind ofcomponents in the ATM switching system, a throughput capacity of them issuppressed to a certain level in view of hardware cost and averageprocessing load, so that even if these processors are increased in theform of a plurality of pairs, there is a limitation as to the throughputcapacity. For this reason, there are required simultaneous processingfor a large amount of calls and signals, when all the terminal unitsconnected to an ATM switching system issue calls at the same time, orwhen a tentative failure in a trunk transmission line is restored. Thus,a processing speed becomes insufficient in the signal processor 106 andthe call processor 108, so that there arises a case where the systemcannot respond to a request by a certain calling subscriber even ifthere is a free speech channel in the ATM switch 101.

Moreover, there is such a high possibility that a calling subscribercalls again after lapse of a certain period of time in the case wherethe ATM switching system did not respond to a call made by the callingsubscriber. Such situation makes processing loads of the signalprocessor 106 and the call processor 108 worse. A countermeasure forsuch situation is to mount such signal and call processors 106 and 108each having a sufficient processing speed. In an ATM switching system towhich a large number of subscriber's terminal units have been connected,however, when it is intended to assure a desired processing speed evenin case where all the subscriber's terminal units were called, anincrease in cost due to an increase in a hardware scale cannot beavoided.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide an ATMswitching system which can respond even if a number of calls are issuedat the same time, besides a cost of which can be reduced.

In order to achieve the above-described object, an ATM (AsynchronousTransfer Mode) switching system for connecting a plurality ofsubscriber's terminal units with a switching network by the use of anATM switch operated in ATM, comprises a call history memory formaintaining call histories of requests for connection from the pluralityof subscriber's terminal units; a reserved connection memory for writingand reading reserved connection information; and a call-signalprocessing section provided with a first means for generating a requestfor connection with respect to the switching network by the use of thecall histories in the call history memory in the case where no call wasissued from the plurality of subscriber's terminal units during apredetermined period of time, and storing contents of a response fromthe switching network with respect to the request for connection in thereserved connection memory as updated reserved connection information,and a second means for using the updated reserved connection informationwhich has been stored in the reserved connection memory to control theATM switch in the case where there was a call from any of thesubscriber's terminal units after applying the first means and therequest for connection is the same as the reserved connectioninformation which has been updated and stored in the reserved connectionmemory.

According to the above-described constitution, the call-signalprocessing section makes a request for connection with respect to aswitching network by the use of a previous call history which has beenstored in the call history memory in the case where no call has beenissued from the respective subscriber's terminal units for apredetermined period of time, and a response result from the switchingnetwork with respect to the request for connection is maintained in thereserved connection memory as reserved connection information.Thereafter, when there was a call from any of the subscriber's terminalunits, contents of the request for connection due to the call arecompared with the response result, which has been maintained in thereserved connection memory, and if both the contents coincide with eachother, processing for connection is carried out by the use of theresponse result, which has been maintained in the reserved connectionmemory (updated reserved connection information). As described above, apreparation for the following call has been completed in the case wherethere was no call from subscriber's terminal units for a predeterminedperiod of time in an ATM switching system of the present invention.Namely, the processing for a call is not started after a request forconnection was received, so that a concentration of load in callprocessing with respect to the request for connection can be reduced ina processing ATM switching system at that time in the case where a callbeing the same as a past (previous) call was issued. Furthermore, aresponse time with respect to the same call can be reduced, so that astart of communication between subscriber's terminal units can be madefaster.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be explained in more detail in conjunctionwith appended drawings, wherein:

FIG. 1 is block diagram showing a conventional ATM switching system;

FIG. 2 is a block diagram showing an ATM switching system according tothe present invention;

FIG. 3 is an explanatory diagram showing a data-retaining format for astoring table in the connection table memory of FIG. 2;

FIG. 4 is a timing chart showing processing timings in the case wherethere is no request for connection from subscriber's terminal units oncalling subscriber side for a certain period of time;

FIG. 5 is a timing chart showing processing timings in the case wherethere was a request for connection from subscriber's terminal units oncalling subscriber side;

FIG. 6 is an explanatory diagram showing examples of generation of timesignals in the clock of FIG. 2;

FIG. 7 is a constitutional diagram showing a constitution of a callhistory region in the call history memory of FIG. 2; and

FIG. 8 is an explanatory diagram showing a data-retaining format of thecall history memory of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be describedhereinafter in conjunction with the accompanying drawings.

FIG. 2 is a block diagram showing an ATM switching system according tothe present invention wherein subscriber's terminal units 2 ⁻¹ to 2_(−n), and a trunk ATM switching network 3 are connected to the ATMswitching system 1, and a subscriber's terminal unit 4 has beenconnected to the trunk ATM switching network 3. The ATM switching system1 is constituted with centering around an ATM switch 11, and aconnection table memory 12 and a call history memory 13 are connected tothe ATM switch 11. A call-signal processing section 15 is connected tothe ATM switch 11, the connection table memory 12, and the call historymemory 13, respectively. A clock 14 is connected to the call historymemory 13, and a reserved connection memory 16 is connected to thecall-signal processing section 15. Furthermore, a timer 17 is connectedto the ATM switch 11 and the call-signal processor 15, respectively. TheATM switching system 1 contains directly the subscriber's terminal units2 ⁻¹ to 2 _(−n), and it functions to demultiplex ATM cells transmittedand received by these subscriber's terminal units to connect them to thetrunk ATM switching network 3.

The ATM switch 11 implements switching of ATM cells transmitted betweenthe subscriber's terminal units 2 ₁ to 2 _(n) and the trunk ATMswitching network 3 in accordance with switch connection information 18written in the connection table memory 12. Moreover, the ATM switch 11outputs requests for connection to be made between the subscriber'sterminal units 2 ⁻¹ to 2 _(−n) and the trunk ATM switching network 3 aswell as responses thereof to the call-signal processing section 15, andfurther it outputs requests for connection 19 from the subscriber'sterminal units 2 ⁻¹ to 2 _(−n) to the call history memory 13 and thetimer 17 as they are as requests for connection 20 from subscriber'sterminal.

The connection table memory 12 stores switch connection information tobe given to the ATM switch 11. The call history memory 13 storescontents of the requests for connection 20 from the ATM switch 11 ineach time. The clock 14 generates time signals 26 to be applied to thecall history memory 13 and the call-signal processing section 15.

The call-signal processing section 15 refers to the call history memory13 in accordance with a reserved connection startup signal 24 from thetimer 17, and outputs a request for connection 25 on the side of theswitching network having the same contents as that of the call historymemory 13 to the ATM switch 11. When the call-signal processing section15 receives the request for connection 20 from subscriber's terminalunits 2 ⁻¹ to 2 _(−n), the contents thereof are compared with that ofthe reserved connection information 23 contained in the reservedconnection memory 16, and as a result, if there is a correspondingelement, the reserved connection information 23 is taken out from thereserved connection memory 16, and the reserved connection information23 thus taken out is retained in the connection table memory 12.

Furthermore, when the call-signal processing section 15 receives arequest for response 22 from the trunk ATM switching network 3concerning the request for connection 21 delivered to the trunk ATMswitching network 3, a VPI (Virtual Pass Identifier)/VCI (VirtualChannel Identifier) value that has not been used yet in a transmissionpath in which a subscriber's terminal unit having a call history residesis selected, the VPI/VCI value is made to be a set with a VPI/VCI valuein a response for connection 27 on the side of a switching networkreceived, and the set of values is retained in the connection tablememory 12 as switch connection information. The ATM switch 11 iscontrolled by the switch connection information thus obtained. Since theswitch connection information is a result obtained by making a requestfor connection in advance with respect to the side of the trunk ATMswitching network 3 to discriminate a theoretical communication path,there is no need to take freshly a confirmation upon the side of thetrunk ATM switching network with respect to a request for connectionfrom the subscriber's terminal units 2 ⁻¹ to 2 _(−n) in the future.

The reserved connection memory 16 executes storage and readout ofreserved connection information 23 in accordance with instructions fromthe call-signal processing section 15. The timer 17 supervises thepresence or absence of reception of the requests for connection 20 fromthe subscriber's terminal units 2 ₁ to 2 _(n), and when no request forconnection 20 arrives within a certain period of time, a reservedconnection startup signal 24 is output to the call-signal processingsection 15.

In accordance with the constitution as described above, when thecall-signal processing section 15 produces a request for connection 25based on the connection request information retained in the call historymemory 13 during a period of time wherein there is no request forconnection 19 from the subscriber's terminal units 2 ⁻¹ to 2 _(−n),processing of the request for connection 19 is started previously, sothat a result of the processing is retained in the reserved connectionmemory 16 as reserved connection information 23. Thereafter, when therequest for connection 19 having the same contents as that of a callhistory from the subscriber's terminal units 2 ⁻¹ to 2 _(−n), thereserved connection information 23 that has been stored in the reservedconnection memory 16 is utilized as it is. Accordingly, there is no needfor starting freshly the other processing. Thus, processing load thathas produced at the time when the request for connection 19 was receivedfrom the subscriber's terminal units 2 ⁻¹ to 2 _(−n), can be reduced.

FIG. 3 is a constitutional diagram showing a storage table in theconnection table memory 12 wherein elements 1 and 2 have been set outwith respect to transmission paths and VPI/VCI values on input andoutput sides viewed from the ATM switch 11 in which a subscriber'sterminal unit to be connected has been assigned to the transmission pathon the input side, while a trunk ATM switching network 3 has beenassigned to the transmission path on the output side. Furthermore,VPI/VCI values have been set out in each element.

FIGS. 4 and 5 are diagrams each illustrating processing timings in anATM switching system according to the present invention. An explanationfor the processing will be mentioned later. In both the figures, acharacter “P” means processing.

FIG. 6 shows examples of generation of time signals 26 of the clock 14.

The clock 14 outputs periodically a time signal 26 indicating a presenttime in accordance with operation of the built-in clock to the callhistory memory 13. In this case, time signals 26 are output once everyhour so as to be 0:00, 1:00, 2:00, . . . wherein signals indicatingtimes (zero o'clock, one o'clock, two o'clock etc.) are output at theoutput timings as shown in FIG. 6 as time signals 26. A memory region ofthe call history memory 13 to which these time signals are to be input(hereinafter referred to as “call history region”) is divided in suchthat data can be independently maintained in every periods extendingfrom a precedent time signal 26 to the following time signal 26, anddata at which time is to be maintained in a certain call history regionhas been previously determined, respectively.

FIG. 7 is a constitutional diagram showing a constitution of callhistory regions of the call history memory 13.

In the call history memory 13, the whole of memory region is dividedequally into twenty-four sections to define call history regions 13 ⁻¹to 13 ⁻²⁴ in order to assure regions in once every hour with respect totime signals 26 each having one hour interval output by the clock 14.The call history memory 13 writes contents of a request for connection20 in a subscriber's terminal unit from the ATM switch 11 in each oftwenty-four call history regions 13 ₁ to 13 ₂₄.

FIG. 8 is a diagram showing a data-retaining format of the call historymemory 13.

In each of the twenty-four call history regions 13 ⁻¹ to 13 ⁻²⁴,contents of the number of the maximum n, i.e., from element 1 to elementn are stored with respect to the contents of request for connectioncomposed of call terminal units (the subscriber's terminal units 2 ⁻¹ to2 _(−n), in the present example), the other end terminal (thesubscriber's terminal unit 4 in the present example), a zone, and atraffic type. The example shown in FIG. 8 represents a situation whereina call history where the time signal 26 extends from “zero o'clock” to“one o'clock” is maintained.

For instance, it is assumed that two requests for connection of CBR(Constant Bit Rate) traffic in an applied zone of 64 kbps derived fromthe subscriber's terminal unit 2 ₁ to the subscriber's terminal unit 4and of VBR (Variable Bit Rate) traffic in an applied zone of 128 kbpsderived from the subscriber's terminal unit 2 ⁻² to the subscriber'sterminal unit 4 are received during a period from after outputting atime signal 26 indicating “zero o'clock” to outputting a time signal 26indicating “one o'clock”. In this case, as shown in FIG. 8, a callterminal, the other end terminal, a zone, and a traffic type are storedin each element as a call history.

As described above, the call history memory 13 maintains successivelycontents of request for connection 20 from the subscriber's terminalunits 2 ⁻¹ to 2 _(−n) transferred from the ATM switch 11 in otheraddresses in the same region before receiving the following time signal26 from the clock 14, and when the following time signal 26 was receivedfrom the clock 14, the call history memory 13 maintains contents of therequest for connection 20 from the ATM switch 11 in a call historyregion assigned to the following time zone. Accordingly, the callhistory memory 13 may be considered to maintain all the contents ofrequests for connection 19 received before the latest twenty-four hours.

In the following, operations of the ATM switching system 1 will bedescribed by referring to FIGS. 2 through 8 wherein the subscriber'sterminal units 2 ⁻¹ to 2 _(−n) communicate with the subscriber'sterminal unit 4 through the ATM switching system 1 and the trunk ATMswitching network 3.

The request for connection 19 (20) from the subscriber's terminal units2 ⁻¹ to 2 _(−n) is always supervised by the timer 17 (P201 in FIG. 4),and when no request for connection 19 (20) is received during a certaincontinual period of time, the processing shown in FIG. 4 is executed.More specifically, when a time signal 26 is delivered from the clock 14,the timer 17 starts up (P202), and it is checked whether or not therequest for connection 20 was received (P203). In the case where anyrequest for connection 20 has not yet been received, the timer 17delivers the reserved connection startup signal 24 to the call-signalprocessing section 15 (P204). The call-signal processing section 15refers to the call history memory 13 (FIGS. 6 and 7 as well as P205) toread out the call history before twenty-three hours (P206), and theresult read is delivered to the ATM switch 11 as a request forconnection 25 (P207). The ATM switch 11 transfers the request forconnection 25 to the trunk ATM switching network 3 as a request forconnection 21 without any modification (P208). Upon transfer of therequest for connection 21, the trunk ATM switching network 3 returns aresponse for connection 22 to the ATM switch 11. The ATM switch 11delivers the response for connection 22 to the call-signal processingsection 15 as a response for connection 27 on switching network side(P210). Contents (VPI/VCI value and the like) of the response forconnection 22 are stored in the reserved connection memory 16 (P211).

On the other hand, when a request for connection 20 was received fromthe ATM switch 11 during supervision by the timer 17, output of thereserved connection startup signal 24 is stopped with respect to thecall-signal processing section 15, and a series of processing shown inFIG. 5 is executed.

In the case when it is intended to call out the subscriber's terminalunit 4 from the subscriber's terminal units 2 ⁻¹ to 2 _(−n), thesubscriber's terminal units 2 ₁ to 2 _(n) deliver a request forconnection 19 to the ATM switch 11 (P301). A format for the request forconnection 19 relates to information of “calling subscriber (any user ofthe subscriber's terminal units 2 ⁻¹ to 2 _(−n))”, “the other endsubscriber”, “applied transmission zone”, “traffic type” and the like ashas been standardized by ITU-T, ATM forum or the like, in other words,it represents a band and quality of communication that is intended toperform by a calling subscriber. Furthermore, it is usual to employVPI/VCI values, which have been previously determined for transmissionand reception of such request for connection and response for connectionas described above in order to distinguish them from the other datacells. For example, values of VPI=0, and VCI=5 are employed. ATM cellsfrom the subscriber's terminal units 2 ₁ to 2 _(n) derived from theVPI/VCI values are transferred to the call-signal processing section 15without any modification. The request for connection 20 is delivered tothe call history memory 13 to maintain therein in the format ofretention shown in FIG. 7 as a call history (P302).

After the reserved connection startup signal 24 was received from thetimer 17 by the call-signal processing section 15, the followingoperations are started. Contents of the request for connection 20residing in a call history region of the following time zone of thepresent time among call history regions of the call history memory 13are first referred to (P303). Then, contents of request such as a callsubscriber, the other end subscriber, an applied transmission zone, anda traffic type, which are written in the request for connection 20, arecompared with the present situation of line in use, and it is judgedwhether or not connection may be permitted (P304).

For instance, when the call-signal processing section 15 receives areserved connection startup signal 24 at the time 00:30 from the timer17, the “one to two o'clock” call history region (the second line memoryregion in FIG. 7) in the call history memory 13 is referred. On onehand, when the call-signal processing section 15 receives the reservedconnection startup signal 24 at the time 01:30 from the timer 17, the“two to three o'clock” call history region (the third line memory regionin FIG. 7) is referred to. In this case, it is to be noted that thehistory contents referred by the call-signal processing section 15corresponds always to that of requests for connection 20 received fromthe subscriber's terminal units 2 ₁ to 2 _(n) in the time zone beforetwenty-three hours with respect to the present time. In the case when“permission” for connection was judged in P304, a request for switchingnetwork connection 25 (=request for connection 21) for transferring tothe trunk ATM switching network 3 is output as a kind of a forecastedvalue or an expected value (P305). The contents of the request forswitching network connection 25 are the same as that of the connectionrequest signal 19, which have been received previously from thesubscriber's terminal units 2 ⁻¹ to 2 _(−n).

The call-signal processing section 15 prepares a connection requestingsignal based on the contents of the forefront element (element 1 in FIG.8), which has been maintained in the call history memory 13 and referredto, to output the same to the ATM switch 11 as a request for connection25 on switching network side, and the request thus output is furtheroutput to the trunk ATM switching network 3 as a request for connection21 (P306). The trunk ATM switching network 3 calculates a routesatisfying a request zone-quality indicated by a request for connection,and a response for connection 22 including a VPI/VCI value to be usedfor communication between the subscriber's terminal units 2 ⁻¹ to 2_(−n) and the subscriber's terminal unit 4 is returned to the ATM switch11 (P307). The call-signal processing section 15 receives the responsefor connection 22 form the trunk ATM switching network 3 through the ATMswitch 11 as a response for connection 27 on switching network side(P308). Then, the call-signal processing section 15 selects a VPI/VCIvalue that has not yet been employed in a transmission path to which asubscriber's terminal unit shown in the element column of thecall-history memory 13 referred to for preparing a request forconnection 25 has been connected, and delivers a response for connection28 including the VPI/VCI value thus selected to the subscriber'sterminal units 2 ⁻¹ to 2 _(−n) from which a request for connection 19has been delivered through the ATM switch 11 (P309). Furthermore, a pairof the VPI/VCI value selected and a VPI/VCI value indicated in theresponse for connection 22 from the trunk ATM switching network 3 iswritten in the reserved connection memory 16 (P313). A data format forwriting data in the reserved connection memory 16 is the same as that ofthe connection table memory 12 shown in FIG. 3. The written informationis used in case of processing the following request for connection.

If a reserved connection startup signal 24 from the timer 17 iscontinued at that time, the call-signal processing section 15 executesthe same processing as that described above with respect to thefollowing element residing in a corresponding region of the call historymemory 13 (“element 2” in FIG. 8). In the case where the reservedconnection startup signal 24 from the timer 17 has been released,processing with respect to the following element is not executed.Thereafter, when a request for connection 20 was received from the ATMswitch 11 (P314), contents of the request for connection 20 are comparedwith that of each element of the reserved connection memory 16 to obtaina prepared value (P315). As a result of the comparison, when there is acoincident element, the element is taken out from the reservedconnection memory 16, the contents thereof are maintained in theconnection table memory 12 as it is (P316). On the other hand, whenthere is no coincident element, it is judged whether or not a requestfor connection can be permitted (P317), and then, a request forconnection 25 is prepared to output it to a switching network side(P318). In the call-signal processing section 15, since the processingP315 can be realized by only retrieval, fresh processing of no use isnot required in the case where there is an element contents of whichcoincide with that of a request for connection 20 (19) received incomparison with the case where there is no coincident element. Hence, aprocessing load of the call-signal processing section 15 decreases,besides a response to a subscriber's terminal unit becomes faster.

The ATM switch 11 executes a line connection between the subscriber'sterminal units 2 ⁻¹ to 2 _(−n) and the trunk ATM switching network 3based on a response for connection 28 (a response for connection 27 onswitching network side) from the call-signal processing section 15(P310). At the same time, the call-signal processing section 15 makes apair of a VPI/VCI value contained in a connection response signalreceived from the trunk ATM switching network 3 and a previous VPI/VCIvalue selected in a transmission path to which a subscriber's terminalunit belonging to a calling subscriber from which the response forconnection 19 was delivered has been connected, and the resulting pairis stored in the connection table memory 12 as switch connectinginformation as shown in FIG. 3 (P312).

As described above, according to the preferred embodiment of the presentinvention, when a request for connection 19 from the subscriber'sterminal units 2 ₁ to 2 _(n) has not yet been received for a certainperiod of time, contents of a request for connection 20 which have beenstored in the call history memory 13 and have been received beforetwenty-three hours form the present time are output to the trunk ATMswitching network 3 as a request for connection 21 without anymodification, and a VPI/VCI value shown in a response for connection 22is stored in the reserved connection memory 16. Thereafter, when arequest for connection 19 having the same contents as that have beenstored in the reserved connection memory 16 are received from thesubscriber's terminal units 2 ⁻¹ to 2 _(−n) there is no need forimplementing freshly processing of a request for connection 21 upon aside of the trunk ATM switching network 3 as well as of the response forconnection 22, processing for confirming requested contents or the like,but contents of the reserved connection memory 16 may simply be againmaintained in the connection table memory 12, because contents of theprocessing result have been already maintained in the reservedconnection memory 16.

As described above, according to an ATM switching system of the presentinvention, a time during which no request for connection is issued fromthe subscriber's terminal units 2 ⁻¹ to 2 _(−n) is utilized, and callprocessing is finished by a call history stored in the call historymemory 13, whereby a processing load in the case where a request forconnection 19 was actually issued from the subscriber's terminal units 2⁻¹ to 2 _(−n) is reduced. Particularly, an advantage of reduction inload appears remarkably in the case where a number of similar calls areissued in the same time zone as that of the previous day.

In the above-described embodiment, it is to be noted that each ofdescriptions as to the connection memory table 12, the call historymemory 13, a data format of the reserved connection memory 16, a cycleand an outputting method of time signals output from the clock 14, and amethod for dividing a maintaining region concerning the call historymemory 13 relates to only an example, and accordingly, the presentinvention is not limited to these manners and methods as describedabove.

As described above, an ATM switching system according to the presentinvention is provided with a call history memory, a connection tablememory, a reserved connection memory, and a call-signal processingsection wherein a request for connection is made with respect to aswitching network by the use of a previous call history which has beenmaintained in the call history memory in the call-signal processingsection in the case where no call has been issued from each ofsubscriber's terminal units during a predetermined period of time, aresponse result from the switching network with respect to the requestfor connection is maintained in the reserved connection memory asreserved connection information, and thereafter, when there was a callfrom a subscriber's terminal unit, contents of the request forconnection due to the call are compared with the response resultmaintained in the reserved connection memory, and it is adapted toimplement processing for connection by the use of the response resultmaintained in the reserved connection memory in the case where both thecontents are the same with each other. Thus, a period of time duringwhich no call is issued from a subscriber's terminal unit for apredetermined time is utilized for completing a preparation of thefollowing call in the ATM switching system of the present invention, sothat a concentration of load for call processing in the ATM switchingsystem can be reduced in the case when the same call as that of the past(previous time) was issued. Besides, a response time for the same calldecreases, whereby starting of a communication between subscriber'sterminal units can be increased.

It will be appreciated by those of ordinary skill in the art that thepresent invention can be embodied in other specific forms withoutdeparting from the spirit or essential characteristics thereof.

The presently disclosed embodiment is therefore considered in allrespects to be illustrative and not restrictive. The scope of theinvention is indicated by the appended claims rather than the foregoingdescription, and all changes that come within the meaning and range ofequivalents thereof are intended to be embraced therein.

1. An ATM (Asynchronous Transfer Mode) switching system for connecting aplurality of subscriber's terminal units with a switching network by theuse of an ATM switch operated in ATM, comprising: a call history memoryfor maintaining call histories of requests for connection from saidplurality of subscriber's terminal units; a reserved connection memoryfor writing and reading reserved connection information; and acall-signal processing section provided with a first means forgenerating a request for connection with respect to said switchingnetwork by the use of said call histories in said call history memory inthe case where no call was issued from said plurality of subscriber'sterminal units during a predetermined period of time, and storingcontents of a response from said switching network with respect to therequest for connection in said reserved connection memory as updatedreserved connection information, and a second means for using saidupdated reserved connection information which has been stored in saidreserved connection memory to control said ATM switch in the case wherethere was a call from any of said subscriber's terminal units afterapplying said first means and the request for connection is the same asthe reserved connection information which has been updated and stored insaid reserved connection memory.
 2. The ATM switching system as claimedin claim 1, wherein: said call history memory is provided with a callhistory region sectioned into one hour each and having an amountcorresponding to twenty-four hours, and subscriber's terminal units toeach of which any call was issued among said plural subscriber'sterminal units, each of the other end subscriber's terminal unitsconnected to said switching network, zones, and traffic types aremaintained in each of sections in said call history region as tables. 3.The ATM switching system as claimed in claim 1, wherein: saidcall-signal processing section is provided with a connection tablememory for storing switch connection information given to said ATMswitch; a clock for outputting periodically time signals for deciding atiming in case of maintaining said call history in said call historymemory; and a timer for delivering a startup signal inducing to refer tosaid call history in said call history memory with respect to saidcall-signal processing section in the case where no call is issued fromsaid plurality of subscriber's terminal units for a certain period oftime.
 4. The ATM switching system as claimed in claim 3, wherein: saidcall-signal processing section refers to said call history that wasstored in said call history memory before a predetermined period of timefrom the present time in the case where said startup signal was receivedby said call-signal processing section from said timer.
 5. The ATMswitching system as claimed in claim 3, wherein: said connection tablememory makes a set of a VPI (Virtual Pass Identifier)/VCI (VirtualChannel Identifier) value, which has not yet been used in a transmissionpath corresponding to a request for connection in said call history ofsaid call history memory, and a VPI/VCI value in a response forconnection from said switching network to store data of the set as saidconnection information.
 6. The ATM switching system as claimed in claim5, wherein: said connection table memory stores said respective VPI/VCIvalues by means of tables corresponding to said plurality ofsubscriber's terminal units and said switching network.